1. Field of the Invention
This invention relates to controlling the performance of liquid-liquid extraction tower processes by adjusting the extraction tower operating conditions in response to the amount of carryunder present in the extract phase recovered from the extraction tower.
2. Related Art
Liquid-liquid solvent extraction is a well established process.
The separation of aromatics from hydrocarbon feed streams comprising mixtures of aromatics and non-aromatics by solvent extraction is a process which has long been practiced in the refining industry especially in the production of lubricating oil. The process involves the use of solvents such as phenol, furfural, n-methyl pyrrolidone which are selective for the aromatic components present in the hydrocarbon feed streams. The hydrocarbon stream and the selective solvents are combined, typically and preferably under countercurrent conditions. The contacting results in concentration of the aromatic component in the selective solvent. Because the solvent and the hydrocarbon oil are of different densities and generally immiscible after the contacting the aromatics rich solvent phase separates from the mixture thereby resulting in an aromatics rich solvent phase, called the extract, and an aromatics lean non-aromatics rich product phase, called the raffinate.
At certain times a portion of the distillate feed to the extractor can become entrained in the continuous extract phase leaving the bottom of the extractor. Entrainment can be caused by excessive total liquid rates (fresh feed and treat solvent) in the tower and/or inordinately high viscosities of the phases due to low temperature and low solvent water content. The tower may be operating without carryunder but after switching to a different distillate stock and adjusting operating conditions carryunder may become prevalent.
The phenomenon is similar to flooding as described in standard chemical engineering texts such as "Liquid Extraction" II Ed by Robert E. Treybal, McGraw-Hill, 1963 page 462.
The presence of carryunder in the extract constitutes loss of yield of desirable product and is a condition to be strenuously avoided. At present, extract is sampled from extraction towers on a regular but infrequent basis.
Because extractions are usually maintained at close to maximum throughput due to economic consideration to operate at maximum charge rates, any variation in feed or solvent quality or characteristic (even in a nominally constant feed or solvent profile operation) can result in the extraction process straying outside optimum limits and carryunder can be encountered.
The maximum throughput of an extractor varies with the nature of the feed charge being processed, the solvent properties (which depend on water content, temperature, pressure, and other operating condition, e.g., tower temperature profile). Unless all potential variable conditions remain constant there will be variation in the maximum throughput.
If the extractor is operating above the maximum throughput, a portion of the charge is physically entrained in the extract solution and leaves the process with the extract. In the case where the extract is the valuable product, this results in contamination of the product e.g. hexane contamination in benzene, which results in a loss valuable product.
In the case where the raffinate is the valuable product, carryunder of feed into the extract constitutes a loss in raffinate yield; that portion of the charge carried under into the extract being downgraded to extract and lost (unless extensive and expensive secondary and/or tertiary raffinate recovery steps are instituted).
If carryunder remains undetected and charge rate and/or recycle to the tower is increased, extractor flooding will occur. This process upset can last several hours even after detection and corrective measures have been taken.
Thus, the need for a fast, repeatable, accurate and continuous means for measuring for carryunder in extraction towers operation exists. It would facilitate operation of extraction tower at maximum throughput and permit more frequent changes in feed charge composition while minimizing the possible of tower operation upset.